Alternating current (AC) electric motors rely on alternating currents passed through induction windings within the stator to cause rotation of the rotor. So-called three phase AC motors include three matched sets of windings positioned radially about the stator. By supplying sinusoidal AC power to each of the sets of windings such that each set receives an alternating current offset by 120 degrees, a largely continuous torque can be imparted on the rotor as it rotates.
Unlike a brushed DC motor, output speed in an AC motor is controlled by the frequency of the current sent to the stator windings. In order to control output torque, and thus speed, a variable frequency drive (VFD) is used to vary the current fed to the AC motor. Because the inductive reactance of the stator windings is proportional to the frequency applied to the winding, increased voltage is necessary to maintain a relatively constant current within the windings, and thus a relatively constant output torque.
In order to properly drive the AC motor, VFD's often operate using a volts/Hz control scheme. In volts/Hz control, the VFD varies the output speed of the motor by supplying AC power to the stator windings at a particular frequency and voltage. For a given desired torque, voltage is proportionally related to the frequency by a so-called “voltage-to-frequency” or “volts/Hz” ratio. By using closed-loop feedback, a VFD using volts/Hz can maintain motor speed in changing conditions. Depending on the configuration of the stator windings, the frequency may result in different output speeds and torques for the rotor.